1. Field of the Invention
The present invention relates to a power transmission apparatus and more specifically to a lubrication structure of a power transmission apparatus for a four-wheel drive vehicle.
2. Description of the Prior Art
In the steering apparatus for automotive vehicles, generally it is possible to improve the stability of the steering operation by always keeping the two front wheel alignment at a toe-in state under any vehicle travelling condition. Here, the toe-in of front wheels implies that each of the two front wheels is inclined inside at the front and outside at the back when the two front wheels are seen from above. To always keep the front wheels at the toe-in condition, irrespective of vehicle bound or rebound conditions, it is necessary to locate a steering rack as low as possible to such a position where a steering linkage becomes flush with a steering knuckle.
In the case of a four-wheel drive vehicle, however, since a transfer case is provided, it is rather difficult to locate the steering rack at a lower position. In more detail, an example of prior-art power transmission apparatus incorporated in a four wheel drive (4WD) vehicle is disclosed in Japanese published Unexamined (Kokai) Patent Appli. No. 63-305032 as shown in FIG. 1. In the drawing, the power transmission apparatus is roughly composed of a center differential gear CD for distributing engine power to front wheels and rear wheels, a front differential gear FD for distributing engine power to right and left front wheels, a viscous coupling VC provided between a power transmit gear PG, and a casing CA of the front differential gear FD.
When an engine is disposed on the vehicle front side, a rotary direction change gear assembly is necessary to convert engine rotative power about a transversal front wheel drive shaft to that about a longitudinal propeller shaft. In the case of the power transmission apparatus shown in FIG. 1, the rotary direction change gear assembly DCG is composed of an intermediate shaft IS, a spur gear SG formed integral with the intermediate shaft IS and in mesh with the power transmit gear PG, a first bevel gear BG1 fixed to the intermediate shaft IS, and a second bevel gear BG2 connected to a propeller shaft and in mesh with the first bevel gear BG1. In the prior-art structure, since the rotary direction change gear assembly DCG is located on the rear side from the front wheel drive shaft, there exist problems in that the size of the power transmission apparatus increases and further the number of parts thereof also increases. In addition, there exists another problem in that it is impossible to locate the steering rack at the lower position because the rotary direction change gear assembly DCG inevitably interferes with the steering rack. Conventionally, therefore, in the case of 4WD vehicles, the steering rack is usually located over the rotary direction change gear assembly DCG, so that the front wheel alignment is not always kept at toe-in state when the vehicle is bounded or rebounded and therefore the steering stability is degraded.
To overcome the above-mentioned problem, it is possible to reverse the above arrangement so that the rotary direction change gear assembly is on the upper side and the steering rack is on the lower side. In this case, however, since the rotary direction change gear assembly is arranged on the upper side and therefore the bearing portion of the gear assembly is inevitably located above the lubricant tank, there exists a problem in that it is impossible to sufficiently lubricate the bearing portion of the gear assembly.
On the other hand, when a large amount of lubricant is reserved so that the bearing portion can be sufficiently immersed in the lubricating oil, the stirring resistance of the lubricant increases markedly, and there exists another problem in that power loss increases.